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Use a heuristic approach to detect infinite state cycles

Count the number of times the loop in P_SetMobjState() executes
and exit with an error once an arbitrary very large limit is reached.

--- a/src/doom/p_mobj.c

+++ b/src/doom/p_mobj.c

@@ -44,6 +44,12 @@

 //

 int test;

+// Use a heuristic approach to detect infinite state cycles: Count the number

+// of times the loop in P_SetMobjState() executes and exit with an error once

+// an arbitrary very large limit is reached.

+

+#define MOBJ_CYCLE_LIMIT 1000000

+

 boolean

 P_SetMobjState

 ( mobj_t*	mobj,

@@ -50,6 +56,7 @@

   statenum_t	state )

 {

     state_t*	st;

+    int	cycle_counter = 0;

     do

     {

@@ -72,6 +79,11 @@

 	    st->action.acp1(mobj);	

 	state = st->nextstate;

+

+	if (cycle_counter++ > MOBJ_CYCLE_LIMIT)

+	{

+	    I_Error("P_SetMobjState: Infinite state cycle detected!");

+	}

     } while (!mobj->tics);

     return true;